1. Field of the Invention
The present invention relates to a method of forming a contact hole in a semiconductor wafer, and more particularly, to a method of forming a contact hole in which the stability of the contact resistance can be enhanced.
2. Description of the Prior Art
In semiconductor processing, a contact hole can be used to form a metal interconnect between devices in the integrated circuit. The contact hole is made by performing a dry-etching process to form a hole on the surface of the wafer. In the prior art method of forming a contact hole, a high etching rate (OX/SiN&gt;15) is applied to proceed a self-aligned contact etching. However, during the etching of the contact hole, polymers are produced and accumulated inside the contact hole, and the higher the etching rate, the more the polymers are produced. When forming a small size contact hole, polymers may be over-accumulated which results in stopping the etch process automatically, or known as "etch stop". Therefore, how to avoid polymers being over-accumulated during the etching process has become a very important issue.
Please refer to FIG. 1 and FIG. 2. FIG. 1 and FIG. 2 are schematic diagrams of the prior art method of forming a contact hole 20 in a semiconductor wafer 10. The semiconductor wafer 10 comprises a silicon substrate 12, a silicon--oxygen layer 14 positioned on the silicon substrate 12, and a photoresist layer 16 positioned on the silicon--oxygen layer 14 which comprises at least one opening 17 extending down to the silicon--oxygen layer 14. The silicon substrate 12 comprises two gate conductive layers 22 with rectangular cross section positioned on the silicon substrate 12, two gate oxide layers 24 positioned between the two gate conductive layers 22 and silicon substrate 12 respectively, two gate cap layers 26 positioned on the two gate conductive layers 22 respectively, and four spacers 28 positioned on the four vertical side walls of the two gate conductive layers 22 respectively. The two adjacent spacers 28 and the surface of the silicon substrate 12 form a groove 27. The opening 17 in the photoresist layer 16 is approximately Positioned above the groove 27.
The method of forming a contact hole of the prior art is performing an anisotropic dry-etching process to vertically remove the silicon--oxygen layer 14 below the opening 17 in the photoresist layer 16. However, there are polymers 29 produced and accumulated during the etching process. The accumulation of the polymers will obstruct the etching process and eventually stop the etching process. Thus, the prior art method can not form the contact hole 20 which extends down to the silicon substrate 12. As a result, contact open may occur and the stability of the contact resistance may be reduced.